A communication network which uses optical fibers is constructed in a core network that connects together metropolises and a metro network that connects together bases in an area. In such a network, a plurality of optical fibers are used in a bundle. Wavelength division multiplexing (WDM) transmission which involves multiplexing a plurality of optical signals having different wavelengths is performed on respective individual optical fibers to realize high-capacity signal transmission (for example, see Non-Patent Literature 1). In order to further increase the transmission capacity, the use of a multi-core fiber (MCF) which is an optical fiber having a plurality of cores instead of an optical fiber (single core fiber: SCF) having one core has been discussed (for example, see Non-Patent Literatures 2 and 3).
In a node of a ring network for wavelength division multiplexing transmission which uses SCF, it is necessary to divide multiplexed optical signals in respective wavelengths in order to add and drop desired signals from optical signals that are multiplex-transmitted through an optical fiber. When a network is configured using MCF instead of SCF in the future, the number of optical signals will increase as the number of transmission cores and the number of signals divided in respective wavelengths will also increase dramatically. Due to this, when a method similar to ADD/Drop in the network which uses SCF is applied to a network which uses MCF, there is a problem that a device for performing Add/Drop of optical signals in each node becomes complex. Moreover, there is another problem that installation and maintenance of nodes take time and labor.